1. Field of the Invention
The present invention relates to an information display device having a coordinate detecting section for use in connection with a television, a laptop computer, a word processor, or the like.
2. Description of the Prior Art
Conventionally, there has been a display-integrated type tablet device having both an image display function and a coordinate detection function (tablet function) as an active matrix type liquid crystal display device as shown in FIG. 8.
The display-integrated type tablet device includes an active matrix type liquid crystal display panel (referred to merely as a "LCD panel" hereinafter) 1, a row electrode drive circuit 2 and a column electrode drive circuit 3 for driving the LCD panel 1, a display control circuit 4 for supplying a display control signal to the row electrode drive circuit 2 and the column electrode drive circuit 3, a detection control circuit 5 for supplying a detection control signal to the column electrode drive circuit 3, a coordinate detection circuit 7 for detecting the coordinates at the tip end of an electronic pen 6 on the LCD panel 1 by receiving a signal from the electronic pen 6, and a control circuit 9 for controlling the coordinate detection circuit 7 and a switching circuit 8 to switch over between an image display operation and a coordinate detection operation.
The LCD panel 1 has a plurality of row electrodes G1, G2, G3, . . . , G6 (an arbitrary row electrode referred to as "G" hereinafter) which are arranged mutually in parallel on a transparent TFT (thin film transistor) substrate 10, and a plurality of column electrodes S1, S2, S3, . . . , S6 (an arbitrary column electrode referred to as "S" hereinafter) which are arranged mutually in parallel and perpendicularly to the row electrodes.
At the intersection of each row electrode G and each column electrode S is provided a TFT 11. Each TFT 11 has its gate electrode connected to the row electrode G and has its source electrode connected to the column electrode S. The TFT 11 has its drain electrode connected to a pixel electrode 12. Pixel electrodes 12 are arranged in a matrix form in areas segmented by the row electrodes G and the column electrodes S.
On a rear side of the TFT substrate 10 is provided an opposite substrate 13 opposite to the TFT substrate 10. On an internal surface of the opposite substrate 13 is provided an opposite electrode 14 in an area approximately identical to that of the TFT substrate 10. Between the pixel electrodes 12 and the opposite electrode 14 are interposed liquid crystals (not shown) to constitute a pixel matrix.
To the opposite electrode 14 is supplied a bias voltage by means of a power supply circuit 16 and an opposite electrode drive circuit 15.
The row electrode drive circuit 2 scans the row electrode G by successively applying row electrode scanning signals g1 through g6 (an arbitrary row electrode scanning signal referred to as "g" hereinafter) (not shown) to each row electrode G on the display panel 1. Then a scanning pulse is applied to the gate electrode of the TFT 11 via the row electrode G to turn on the TFT 11.
The column electrode drive circuit 3 applies column electrode drive signals s.sub.1 1 through s.sub.1 6 (an arbitrary column electrode drive signal referred to as "s.sub.1 " hereinafter) (not shown) having a drive pulse corresponding to the display contents of the pixel relevant to each row electrode G to column electrodes S in synchronization with the scanning of the row electrode G. Then a drive pulse is applied to the source electrode of each TFT 11 via the column electrode S, with which a signal voltage is applied to the pixel electrode 12 connected to the drain electrode of the TFT 11 which has been turned on by the scanning of the row electrode G by the row electrode drive circuit 2 in a manner as described above. Thus an image is written into each pixel according to the contents of display image.
The column electrode drive circuit 3 scans the column electrode S by successively applying scanning pulses of column electrode scanning signals s.sub.2 1 through s.sub.2 6 (an arbitrary column electrode scanning signal referred to as "s.sub.2 " hereinafter) (not shown) to the column electrodes independently of the scanning of the row electrode G.
It is to be noted that the scanning pulses generated by the row electrode drive circuit 2 and the drive pulses and scanning pulses generated by the column electrode drive circuit 3 are generated based on a bias voltage from the power supply circuit 16.
The display control circuit 4 generates a display control signal for displaying an image on the LCD panel 1 based on a display data signal and a synchronization signal input externally. Then the generated display control signal is transmitted to the row electrode drive circuit 2 to control the operation of the row electrode drive circuit 2. Furthermore, the display control signal and the input display data signal are transmitted to the switching circuit 8. Meanwhile, the detection control circuit 5 generates a detection control signal for detecting the coordinates at the tip end of the electronic pen 6 based on the synchronization signal, and transmits the signal to the switching circuit 8.
The switching circuit 8 switchingly selects the display control signal and display data signal from the display control circuit 4 or the detection control signal from the detection control circuit 5 based on a switching signal from the control circuit 9, and transmits the selected signal to the column electrode drive circuit 3. Thus the operation of the column electrode drive circuit 3 is controlled by the display control signal or the detection control signal.
The electronic pen 6 has at its tip end a detection electrode (not shown) having a high impedance electrostatically coupled with the row electrode G and the column electrode S on the display panel 1. An induction voltage is induced at the detection electrode due to a scanning pulse applied to the row electrode G or a scanning pulse applied to the column electrode S. The coordinate detection circuit 7 detects a generating timing of the induction voltage induced at the detection electrode of the electronic pen 6 based on a coordinate detection timing signal from the control circuit 9 to detect the coordinates at the tip end of the electronic pen 6.
FIGS. 9A and 9B are charts showing the operation timing of the above-mentioned display-integrated type tablet device. One cycle Tv of a vertical sync signal V that is one of the aforementioned synchronization signals is divided into a one-frame image display period (y-coordinate detection period) T1 and an x-coordinate detection period T2, where the x-coordinate detection period T2 is equal to a vertical blanking period Tb. The division of the one cycle into the image display period (y-coordinate detection period) T1 and the x-coordinate detection period T2 is executed by switchingly selecting between the display control circuit 4 and the detection control circuit 5 by means of the switching circuit 8.
Unfortunately, the aforementioned conventional display-integrated type tablet device has the following problems.
A first problem will now be described. According to the operation timing of the display-integrated type tablet device, there is required a time corresponding to one cycle Tv of the vertical sync signal V in order to detect the coordinates at the tip end of the electronic pen 6 on the display panel 1. The reason for the above is that, in detecting the coordinates at the tip end of the electronic pen 6, the column electrode scanning signals s.sub.2 applied successively to the column electrodes S in the vertical blanking period Tb are utilized to detect the x-coordinate value, and the row electrode scanning signals g applied successively to the row electrodes G in the image display period T1 are utilized to detect the y-coordinate value. The above-mentioned arrangement allows the coordinates at only one point to be detected in one cycle Tv of the vertical sync signal V.
According to the above-mentioned arrangement, when the frequency of the vertical sync signal V is 60 Hz, detection of coordinates at 60 points per second is permitted. However, when coordinate detection at a faster speed is required, for example, in the case of inputting an alphabet character in longhand, a reduced character recognition accuracy or impossible character recognition results.
FIG. 10 shows an exemplified image display in the case where a character "a" is input in longhand. In the present case, the movement speed of the electronic pen 6 is so fast that the coordinate detection speed cannot catch up with the input speed. Therefore, only a reduced number of points P of a series of points constituting the character "a" are displayed as shown in FIG. 10, and therefore the series of points displayed cannot be recognized as the character "a".
Furthermore, in the case of a display device capable of detecting input coordinates other than the display-integrated type tablet device, so long as the input coordinates are detected in a specified cycle, detection of the input coordinates in a cycle faster than the specified cycle cannot be achieved though there is a difference depending on the adopted coordinate detection method.
A second problem will now be described. In the aforementioned display-integrated type tablet device, the x-coordinate value at the tip end of the electronic pen 6 is detected by successively applying a scanning pulse of the column electrode scanning signal s.sub.2 to each column electrode S in the vertical blanking period Tb. In the above case, there is a time lag or propagation delay in the transmission of the scanning pulse through the applied column electrode scanning signal s.sub.2 at the leading end of each column electrode S. When the time lag or propagation delay is great, a degraded coordinate detection accuracy results.
When a time margin is in the vertical blanking period Tb, the column electrode scanning signal s.sub.2 can scan each column electrode S at a speed slower than the scanning speed of the column electrode drive signal s.sub.1 in the image display period T1, and therefore the x-coordinate detection accuracy can be increased by correcting the detected coordinate value. However, since a specified time is practically consumed in the image display period T1, there is no time margin for effecting the correction in the vertical blanking period Tb, and therefore the conventional coordinate detection accuracy has not been good.